Porous hollow fiber membranes consisting of polyethylene and having a membrane structure formed from stacked rectangular pores are conventionally known, and the details thereof are disclosed, for example, in Japanese Patent publication Nos. 35726/'88 and 42006/'88. It is disclosed in the former that, when a high-density polyethylene having a melt index of 1 to 15 and a density of 0.960 g/cm.sup.3 or greater is melt-spun at a spinning draft of 1,000 to 10,000, the spun hollow fibers are cold-stretched at a stretching speed of not less than 50% per second, and the cold-stretched hollow fibers are hot-stretched at a temperature of 80 to 125.degree. C. until a total amount of stretching of 400 to 700% is attained, the resulting porous hollow fiber membranes have characteristic rectangular pores, an average pore diameter of 0.5 to 2 .mu.m as measured with a mercury porosimeter, a porosity of 30 to 90% by volume, and a blue dextran rejection of less than 90%. However, the average pore diameters of the porous hollow fiber membranes described in the Examples of this patent are at most 0.82 .mu.m. In Japanese Patent Publication No. 42006/'88, it is disclosed that, when polyethylene is melt-spun at a spinning draft of greater than 2,000 with a spinning cylinder, 5 to 30 cm long, installed beneath the spinneret, and the spun hollow fibers are stretched until a total amount of stretching of 100 to 400% is attained, the resulting porous hollow fiber membranes have a water permeability of 100 to 2,000 l/m.sup.2 .multidot.hr.multidot.760 mmHg, a human serum albumin permeability of not less than 30%, and a blue dextran rejection of not less than 90%. Thus, the hollow fiber membranes provided by the latter patent have a smaller pore diameter than those provided by the former patent.
Moreover, it is disclosed in Japanese Patent Laid-Open No 86902/'86 that porous hollow fiber membranes having pore diameter of 0.1 to 1.0 .mu.m are produced by forming pores according to the stretching method. These porous hollow fiber membranes are characterized by the fact that the microfibrils oriented in the lengthwise direction of the fibers are broken to a substantial degree. Thus, the pore diameter is increased by breaking the microfibrils to expand the rectangular pores in the lateral direction. As a matter of actual fact, however, the resulting porous hollow fiber membranes have a pore diameter of at most 10 .mu.m.
Furthermore, a hydrophilic composite porous membrane consisting of a porous matrix formed of a polyolefin and a coating layer formed of an ethylenevinyl alcohol copolymer is disclosed in Japanese Patent Laid-Open No. 271003/'86. It is described in the claims of this patent that its average pore diameters is within the range of 0.02 to 4.0 .mu.m. With respect to pore diameter, however, the technique disclosed in this hydrophilic composite porous membrane never exceeds the technical level of the aforementioned Japanese Patent Laid-Open Nos. 35726/'88 and 86902/'86. In fact, the hollow fiber membranes obtained in the Examples of that patent have an average pore diameter of 0.25 to 0.70 .mu.m.
Thus, porous hollow fiber membranes consisting of polyethylene and having rectangular pores whose average diameter exceeds 1.0 .mu.m have not been available in the prior art.
Generally, porous membranes are broadly divided into hydrophilic membranes and hydrophobic membranes according to the properties of the material. Known examples of hydrophilic porous membranes include cellulose, cellulose derivatives, polyvinyl alcohol, ethylenevinyl alcohol copolymers and the like. Hydrophilic porous membranes are characterized by the fact that, since the pore surfaces thereof are hydrophilic, they are easily wettable with water and permit the filtration of aqueous solution without any special pretreatment.
However, hydrophilic membranes have the disadvantage that, when they are in the wet state, they show a reduction in mechanical strength and that they undergo a high degree of swelling with water. Moreover, hydrophilic membranes have the additional disadvantage that, when they are dried from the wet state, they are reduced in membrane properties and are liable to deteriorate.
On the other hand, when hydrophobic porous membranes are used as they are, water cannot easily permeate therethrough. Accordingly, a hydrophilizing treatment is required in order to enable hydrophilic liquids including water to permeate therethrough. A variety of methods have been studied especially in connection with hydrophilization by surface modification of polyolefin membranes. It is to be noted, however, that hydrophilizing methods which have been proposed for film-like materials having smooth surfaces cannot be simply applied to the hydrophilization of porous membranes having complex surface configurations.
Well-known methods for hydrophilizing porous polyolefin membranes include the organic solvent wetting and water substituting method in which the entire surfaces, inclusive of pore surfaces, of a porous polyolefin membrane are subjected to a wetting treatment with an organic solvent having good miscibility with water, such as an alcohol and ketone, followed by substitution of water for the organic solvent; the physical adsorption method in which a hydrophilic material such as polyethylene glycol or a surfactant is adsorbed on the surfaces of a porous membrane to impart hydrophilicity to the porous membrane (Japanese Patent Laid-Open Nos. 153872/'79 and 24732/'84); and the chemical surface modification methods in which a hydrophilic monomer is held on the surfaces of a porous film and then exposed to radiation (Japanese Patent Laid-Open No. 38333/'81) or a porous structure consisting of a hydrophobic resin is subjected to a plasma treatment (Japanese Patent Laid-Open No. 157437/'81).
In the organic solvent wetting and water substituting method, however, it is necessary to keep water around the porous membrane at all times and, therefore, its handling is troublesome. The reason for this is that, once water is lost from pores during storages or use, the part having such water-free pores regains hydrophobicity and no longer permits the permeation of water therethrough. Although the physical adsorption method is simple in operation, the hydrophilic material may come off while the porous membrane is used for a long period of time. Therefore, this method cannot be regarded as a fully satisfactory hydrophilizing method. Moreover, the conventional chemical surface modification methods are also disadvantageous. Whether a porous membrane is exposed to radiation or subjected to a plasma treatment, it is difficult to hydrophilize the membrane uniformly in the direction of its thickness. If it is attempted to effect uniform hydrophilization over the entire thickness of a porous membrane which has a large thickness or is in the form of hollow fibers, the matrix of the porous membrane is unavoidably damaged or reduced in mechanical strength.
It has also been proposed to hydrophilize a hydrophobic porous membrane by previously treating it with a saponification product of an ethylene-vinyl acetate copolymer, i.e., an ethylene-vinyl alcohol copolymer (Japanese Patent Laid-Open Nos. 125408/'86 and 271003/'86).
It has also been proposed to produce hydrophilic porous hollow fiber membranes having hydrophilized pore surfaces, by melt spinning a blend of two different polymers, stretching the spun fibers to cleave the interfaces between the different polymers, and then subjecting the resulting microporous hollow fibers to an after-treatment such as hydrolysis or sulfonation of the side-chain groups present in the constituent polymers (Japanese Patent Laid-Open No. 137208/'80).
In addition, there have been proposed porous polyolefin membranes having a hydrophilic polymer held firmly on the pore surfaces thereof, and a production process thereof (Japanese Patent Laid-Open No. 190602/'88). More specifically, this patent provides hydrophilic porous membranes comprising porous polyolefin membranes having a hydrophilic crosslinked polymer, which is composed of monomers including diacetone acrylamide and a crosslinkable monomer, held on at least a part of the pore surfaces thereof. Such hydrophilic porous membranes can be produced by holding monomers including diacetone acrylamide and a crosslinkable monomer on at least a part of the pore surfaces of a porous polyolefin membrane, and then polymerizing them by the application of heat.
However, since all of these methods use a conventional porous polyolefin membrane as the starting material, the performance of the resulting hydrophilized porous membranes is insufficient for fields of application requiring a high permeation flow rate. That is, in order to obtain a hydrophilized porous membrane having a high permeation flow rate, an increase in the pore diameter and porosity of the starting hydrophobic porous membrane needs to be considered in combination with hydrophilization.
In the fields of precision filtration and air cleaning which require a very high filtering flux, membranes and nonwoven fabrics having a pore diameter of the order of microns and a high porosity are being used. Porous hollow fiber membranes having rectangular pores are characterized by the fact that, as described in Japanese Patent Publication No. 35726/'88, they have high permeability to gases and liquids and they are less liable to clogging because of their membrane structure formed from stacked rectangular pores. However, the performance of such porous hollow fiber membranes having rectangular pores is still insufficient for fields of application requiring a very high filtering flux and a low pressure loss, such as bacteria-free and dust-free air filters, dust-removing filters for various gases, and sterile water filters. The reason for this is that their pore diameter is too small and their porosity is too low for use in such fields.
On the other hand, hydrophilizing treatment with an alcohol or a surfactant provides only temporary hydrophilization. Moreover, if a porous membrane having such a hydrophilizing agent attached thereto is used for filtering or other purposes, the alcohol or surfactant migrates to and contaminates the purified water. Accordingly, it is necessary to wash off the hydrophilizing agent thoroughly before use. On this occasion, the pore surfaces regain hydrophobicity if the porous membrane is dried. Thus, once the porous membrane is hydrophilized, it is necessary to replace the hydrophilizing agent with water and thereby keep its pore surfaces in contact with the water at all times.
The method descried in Japanese Patent Laid-Open No. 38333/'81 can provide permanent hydrophilization, because the groups developing hydrophilicity are chemically fixed to the porous membrane. However, the need of exposure to ionizing radiation requires large-scale equipment, provides rather low process stability, and involves a risk of causing damage to the membrane material. Thus, it is difficult to manage and control the process steps.
In the method described in Japanese Patent Laid-Open No. 137208/'80, the hollow fiber membranes produced by melt-spinning a blend of different polymers and then stretching the spun fibers to make them porous generally have a low porosity. Moreover, this method requires an after-treatment for hydrophilization, such as hydrolysis or sulfonation, which makes the process complicated.
Moreover, even if the technique of Japanese Patent Laid-Open No. 38333/'81 is applied to the hollow fiber membranes of Japanese Patent Publication No. 35726/'88, the resulting hydrophilized hollow fiber membranes have only a submicron pore diameter. With respect to pore diameter, the techniques disclosed in the hydrophilic composite porous membranes of Japanese patent Laid-Open Nos. 125408/'86 and 271003/'86 never exceed the technical level of the aforementioned Japanese Patent Publication No. 35726/'88 or the like. In fact, the hollow fiber membranes obtained in the Examples of those patents had an average pore diameter of 0.25 to 0.70 .mu.m.
Furthermore, Japanese Patent Laid-Open No. 190602/'88 discloses hydrophilic porous membranes comprising porous polyolefin membranes having a hydrophilic crosslinked polymer, which is composed of monomers including diacetone acrylamide and a crosslinkable monomer, held on the pore surfaces thereof.
However, the performance of these porous membranes having a crosslinked polymer held on the surfaces of rectangular pores is still insufficient for fields of application requiring a very high filtering flux and a low pressure loss, such as the filtration of aqueous solutions and aqueous suspensions, the preparation of purified water for use in electronic industry and the like, and the sterilization of raw water for use in the manufacture of pharmaceutical preparations. The reason for this is that its pore diameter is too small and its porosity is too low for use in such fields. If permanent hydrophilicity could be imparted to a porous hollow fiber membrane exhibiting a membrane structure formed from stacked rectangular pores capable of providing excellent filtering efficiency and, moreover, having a large pore diameter and a high porosity, great contributions would be made to frontier industrial fields through achievement of energy saving and creation of an ultraclean environment.